WASHINGTON — After a suspected gunman was accused of killing 17 people at a Florida school with a legally purchased AR-15 rifle, one of President Trump's early tweets about the horrific event focused not on gun control, but on mental health.

"So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behavior," Trump wrote Thursday morning on Twitter. "Neighbors and classmates knew he was a big problem. Must always report such instances to authorities, again and again!"

So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behavior. Neighbors and classmates knew he was a big problem. Must always report such instances to authorities, again and again!

What he didn't mention: In one of his early acts as president, Trump signed a measure passed by the Republican-led Congress that repealed an Obama-era regulation designed to block some mentally ill people from buying guns.

The reversal of the Obama regulation — which would have required the Social Security Administration to report the records of some mentally ill beneficiaries to the FBI's background check system — was one of just a string recent legislative victories for the powerful National Rifle Association.

Last March, for instance, the House passed a measure that would block the Department of Veterans Affairs from reporting veterans' records to the background check system after they've been determined incapable of managing their affairs because of a mental disability. The Senate hasn't taken up the bill yet

And just this week, Trump's 2019 budget proposal to Congress proposed a 16% cut in federal grants to help states boost their reporting of criminal records and protection orders to the national database used for background checks on gun purchasers.

NRA officials did not immediately respond to an interview request Thursday,

Those Washington wins come after record spending in the 2016 election by gun-rights forces.

The National Rifle Association and other gun-rights organizations spent nearly $55 million in the 2016 election cycle to oppose or support candidates through independent spending — nearly 19 times the amount spent by groups promoting gun restrictions, according to a tally be the nonpartisan Center for Responsive Politics.

Trump was one of the biggest beneficiaries of the NRA's spending in the last election, with the group pumping more than $31 million into advertising to boost his candidacy and to attack his Democratic rival, Hillary Clinton. The spending was remarkable because the NRA was the largest, well-established outside group to back Trump's unorthodox campaign.

In addition, more than $61 million in NRA money has gone to back current members of Congress, most of which benefited Republicans. Topping the list: Arizona Sen. John McCain, a former GOP presidential nominee, who benefited from $7.7 million in NRA spending over his political career, according to the Center's data.

Bess Kalb, a writer for Jimmy Kimmel Live! has thrown a fresh spotlight on NRA's political spending this week, responding to lawmakers who are offering "thoughts" and "prayers" to the victims of Wednesday's shooting in Florida, with tweets showing how much members of Congress have benefited from the group's political spending.

The NRA's use of independent spending — on ads, mailers and emails — to mobilize voters has made it "one of the most powerful political organizations in America," said Adam Winkler, a UCLA law professor and author of 2011 book, Gunfight: The Battle Over the Right to Bear Arms in America.

"Politicians don't listen to the NRA simply because they spend money, they listen to the NRA because the NRA's money is effectively spent swaying voters," he said.

Sheila Krumholz, of the Center for Responsive Politics, said it's not just money that gives the NRA influence, but its membership of nearly five million activists.

"The money is a megaphone for another significant asset: a powerful base," she said.